Chain a multiple of 15 + 2
Row 1: dc in 4th ch from hook, dc in each of next 2 ch, *[skip next 3 ch, (4 dc, ch 1, 4 dc) in next ch (shell made), skip next 3 ch], dc in each of next 8 ch, rep from * across to last 11 ch, rep between [ ] once more, dc in last 4 ch, ch 3, turn.
Row 2: dc in each of next 3 sts, *[ch 3, skip next 4 sts, sc in next ch 1 sp, ch 3, skip next 4 sts], dc in each of next 8 sts, rep from * across to last shell, rep between [ ] once more, dc in last 4 sts, ch 3, turn.
Row 3: dc in each of next 3 sts, *[skip next ch 3 sp, (4 dc, ch 1, 4 dc) in next st, skip next ch 3 sp], dc in each of next 8 sts, rep from * across to last shell, rep between [ ] once more, dc in last 4 sts, ch 3, turn.
Row 4: dc in each of next 3 sts, *[ch 3, skip next 4 sts, sc in next ch 1 sp, ch 3, skip next 4 sts], dc in each of next 8 sts, rep from * across to last shell, rep between [ ] once more, dc in last 4 sts, ch 3, turn.
Repeat rows 3 and 4 to desired size.
You can see my previous blog post about this stitch pattern here.